In process plants, field devices that sense or control parameters for processing chemicals and pharmaceuticals are smart devices. “Transmitter” is often synonymous with smart device. A smart device is a measurement and/or control module with a transmitter that can communicate with host devices over a wired or wireless communication channel using one or more standard protocols.
Host devices include computers, calibrators, communicators, tablets and personal digital assistants. Some standard protocols include HART, an acronym for Highway Addressable Remote Transducer, or FOUNDATION Fieldbus. HART is an industry standard for a hybrid protocol that enables communications between smart field devices and a control system that employs legacy 4-20 mA wiring. FOUNDATION Fieldbus is an all-digital protocol.
A smart device with HART protocol has a microprocessor and one or more memories. The memories hold data about the manufacturer of the device, its current readings for a primary variable and other variables, and its ranges, as appropriate for variables. The microprocessor is programed to perform a number of specific operations and to respond to predetermined commands. Such commands include and are not limited to reading manufacturing information and reading a primary variable (e.g., pressure) and one or more secondary variables (e.g., temperature). Other commands are used to calibrate the smart device and include setting the range of the measured variables, including their upper and lower range limits.
A smart device has one or more transducers that measure physical parameters, such as pressure or temperature or whether a valve is open or closed. Transducers convert a signal of parameters in one form of energy (e.g., pressure, flow) into electrical signals. The output of the transducer is converted from analog to digital values by an analog-to-digital converter whose output is input to a microprocessor. Some smart devices may have a microprocessor and circuitry for performing A-to-D and D-to-A conversion in a single chip. Depending upon the type of communication network, after processing, the microprocessor output may be converted into an analog value for transmission via a wired or wireless transmitter.
Each smart device is associated with a Device Description. A Device Description (DD) is an electronic data file describing the capabilities of a smart device, defining a hierarchical structure of read-only parameters, read/write parameters, standard operating procedures (methods) and other information for use by a communication host. A DD has menus and graphic display features that are used by host applications (including handheld calibrators, communicators and other hosts) to access all parameters and data in the corresponding smart device. A DD and its menu structure are unique to a device and a device revision. A DD for one device cannot be used with a completely different device from another manufacturer, even if that other device were to offer the same functionality. However, DDs do support multi-vendor interoperability. Each device may be calibrated to specific parameters so that process managers may configure transmitters from one manufacturer to replace a device from another manufacturer. The DD does not contain any information about current values in a smart device but enables a host to read the memory of the device and to acquire items of device information that are stored in the device transmitter. Device information includes tag information given by a user to the device to identify its location or function and information identifying the manufacturer, model, and revision of the device. It may also include the units of the primary variable of the device, the lower range value, upper range value, damping, and transfer functions. Many devices have linear transfer functions but some have square root transfer functions, or other non-linear transfer functions.
The DDs allow the process operator to set hundreds of device variables in the smart device. Process operators use DDs to integrate smart devices of different manufacturers into a process. However, a technician calibrating a device needs access to a relatively small number of device variables that are set in the smart device. Nevertheless, access to those device variables is only possible by following the tree-structure of the DD.
The DD is held in a memory of the calibrator or communicator. A DD comprises multiple levels and is arranged in the tree-like structure with branches between levels. A DD is similar to a database of information about the device, and includes algorithms for use when attempting to accomplish specific tasks with the device, such as sensor trim, and locations for storing parameters for the smart device. DDs provide the information needed by a host such as a communicator, calibrator, host application, or control system to properly access and display important device information located in smart devices.
A communication host uses a DD to communicate with a smart device. A communication host is capable of communicating with smart devices using one or more protocols with the ability to display various parametric values and allowing the user to set configuration parameters on such devices. Communication hosts include and are not limited to central controllers, calibrators and communicators. A host may use DDs that persist in the host's storage memory in order to provide access to the full capabilities, as described by the provider of the DD, of that smart device.
The DD is an electronic data file describing the capabilities of a smart device, defining a hierarchical (tree) structure of read-only parameters, read/write parameters, standard operating procedures (methods) and other information for use by a host. A DD is typically written by the manufacturer of a smart device. Specifications for the authoring and structure of DDs are controlled by standard bodies such as IEC and ISA with contributions from HART Communications Foundation, Fieldbus Foundation and PROFIBUS and PROFINET International. It declares or explains a device such as a smart device transmitter. Actual processing is done by communication host devices such as documenting calibrators.
The tag is stored in the memory of the smart device, not in the DD. The tag parameter/field is entered in the smart device by the end user. It is typically, but not necessarily always, used to name the device as well as identify its location or function in a plant. A host, such as a calibrator or communicator uses universal commands to locate and read tag information. A host also reads the DD and navigates via the DD tree to locations in the DD that hold, its primary, secondary, and other variables and its parameters.
Smart devices used in process control undergo aging, wear, and tear. It is best practice to periodically inspect smart devices to see whether they have maintained their desired settings and to calibrate or adjust the smart devices that are out of calibration tolerance. Calibration is a process for verification (test) and adjustment (trim) of a smart device. Process calibrators store calibration specification and configuration information that can be recalled for use during verification, adjustment and for other uses. This information is referred to as calibration setup and includes, but is not limited to:                1. Steps for calibration as a sequence of operations and conditional checklists for execution during calibration;        2. The parameter for the calibrator to measure (electrical, physical or digital) during verification;        3. The parameter for the calibrator to source (electrical or physical) during verification;        4. The test range for the verification in the form of 100% (upper test range) and 0% (lower test range) values;        5. The test specification (tolerance) for the verification; and        6. The test strategy for the verification, including the number of test points, sequence and distribution of those test points and the transfer function.        
Verification is a process of comparing one or more outputs of a smart device with corresponding standard inputs against the test specifications. There are two stages to verification: (1) As Found is the calibration state of the smart device at the start of the calibration process; and (2) As Left is the calibration state of the smart device at the end of the calibration process. Sometimes, the As Left stage is skipped if the As Found verification confirms that the smart device is within calibration specifications and no adjustment was performed.
During the As Found stage, a technician tests the input, the transfer function and the output of the smart device at the low range value, the high range value and one or more points between low and high range values and records the results as the As Found results. If any result is out of range of tolerance, the technician uses a communicator and/or calibrator to perform one or more iterations of adjustment to achieve results as close as practical to tolerance. After an adjustment is made, the As Left verification is performed to confirm that the results are again within specification. The adjustment and As Left verification steps may be repeated multiple times as needed.
Calibrators verify and adjust smart devices by supplying standard inputs into devices at the required accuracy during the process of calibration. This is commonly referred to as a “source” from the calibrator. The inputs sourced may be known values of electrical or physical parameters, including and not limited to voltage, current, pressure, and temperature. A calibrator also provides the capability to measure output from the device under calibration. The parameters being measured may be electrical, physical or digital.
Calibration includes the capability to adjust the zero and gain to set a desired range. In order to calibrate a smart device, a highly precise and known standard measured quantity, such as a known pressure or a known temperature, is compared to the output reading of the device. Sometimes an electrical value (ohms or millivolt) is applied to the transmitter, when the transmitter measures temperature using an RTD or thermocouple. If the transmitter in the smart device indicates a value different by more than an allowable tolerance from the known standard, the transmitter is adjusted (trimmed) and then retested so that its output corresponds to the standard or within tolerable limits thereof.
Typically, calibration of a variable monitored by a smart device is checked at several points in the prospective calibrated span of the variable. Span is defined as the difference between the upper and lower range values. The zero value is the lower limit of the span range and the range is the difference between upper and lower range values. The calibration range (span) often differs from the operating range, which refers to the capability of the device.
For example, an electronic pressure transducer in a smart device may have a range capability of 0-600 psi and output of 4-20 milliamps (mA). However, a process engineer may determine the smart device will be calibrated for 50-200 psi=4-20 mA. Therefore, the calibration range would be specified as 50-200 psi=4-20 mA. In this example, the zero input value is 50 psi and zero output value is 4 mA. The input span is 150 psi and the output at the upper limit is 20 mA.
Smart devices are typically calibrated to show a linear, square root, or other transfer relationship between input and output. An ideal graph of a linear transfer relationship in a calibrated smart device is an input-output curve that is a straight line beginning at the origin of the X and Y axes and having a predetermined slope. Zero error is corrected by parallel shifting the input-output characteristic curve of the smart device; span error is adjusted by changing the slope of the curve.
Some calibrators are process calibrators and are capable of measuring and sourcing parameters most commonly used in process industries. Some process calibrators are documenting process calibrators (DPC). A DPC, in addition to providing the capabilities of a process calibrator, provides the ability to electronically record (i.e., document) the results of a calibration. DPCs that are capable of communicating with smart devices combine the functionality of a portable calibrator with that of a handheld field communication host into a single tool. DPCs may have additional capabilities such as the ability to transfer data from a personal computing device and/or create and store calibration setup information for later recall and use during calibration.